[jdk17u] RFR: 8268261: C2: assert(n != __null) failed: Bad immediate dominator info.

[Vladimir Kozlov]

Backport of 8268261. Applied cleanly.
Reviewed-by: iveresov, vlivanov
Tested for jdk17u: tier1

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Commit messages:
 - 8268261: C2: assert(n != __null) failed: Bad immediate dominator info.

Changes: https://git.openjdk.java.net/jdk17u/pull/6/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17u&pr=6&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268261
  Stats: 75 lines in 2 files changed: 74 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17u/pull/6.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17u pull/6/head:pull/6

PR: https://git.openjdk.java.net/jdk17u/pull/6